This invention relates to an improved circuit arrangement for a keyboard of the type in which the switches are connected in series with diodes, forming a switch matrix, the matrix rows being connected to a multiplexer and the matrix columns being connected to an evaluation device.
Circuit arrangements of this type are known (U.S. Pat. No. 4,033,221 entitled, "Key Switch System", Hamamatsu et al) in which the individual matrix rows are called up one after the other by the multiplexer and communicate the switch states of the switches to a memory. The diodes which are connected in series with the switches prevent reciprocal influence of the matrix columns. It is also known (German Auslegeschrift No. 27 08 212 published Nov. 24, 1977) how to switch LED's into a matrix form for purposes of indication of positions, and how to activate one row and one column of the diode matrix by a decoder, so that the LED's light up at the point of intersection of the relevant row and column. Finally, a circuit with operating keys for switching on device operations is known (U.S. Pat. No. 4,083,012 entitled "Automatic Function Setting and Communication Arrangement For a Communications Receiver", W. Kamow), in which each operating key is associated with an LED, which lights up after activation of the operating key.
The invention is directed to creating a circuit arrangement of the type initially mentioned, in which the commands entered into the keyboard, or other information, are optically indicated without noteworthy increased expense. A preferred embodiment of the invention is explained below in more detail with the aid of a diagram.